A liquid-crystal display (LCD) may be considered a variable light filter. Liquid crystals are trapped between two sheets of glass and walled off from one another into image bits called pixels. These crystals twist and untwist to let polarized light through, and filters placed in front of the pixels create the colors that emerge. Since an LCD only modifies light and does not create it, the quality of an LCD is dependent on the spectrum of light emerging into it from the rear, which is hereinafter referred to as backlighting.
A cold-cathode fluorescent lamp (CCFL) is often used for backlighting an LCD. One problem that often occurs with the use of a CCFL is that a relatively large amount of power is required to operate the CCFL. Another problem connected with the use of a CCFL is that a CCFL only produces an approximation of white light, not true white light. Since an LCD can only make color within the spectrum of the light it receives, a CCFL-based LCD has a color gamut (the extent of the mix of color a display is capable of producing) smaller than for example, a cathode-ray tube (CRT) or a plasma display. The CRT and the plasma display use energized phosphors to create purer colors.
An LED (Light Emitting Diode) array may also be used for backlighting an LCD. One advantage of using an LED array for backlighting an LCD is that the LED array is capable of generating greater spectral accuracy than a CCFL. For example, mixing the appropriate amount of light from red, green, and blue LEDs will produce white light. In addition, since the color balance of the white balance can be adjusted (by varying the relative intensity of the red, green, and blue LEDs), the color balance of an LCD may be maintained reasonably accurately over the operating life of the LCD. Another advantage of using an LED array for backlighting is that an LED array may provide better color saturation.
LEDs are generally not as fragile as CCFLs and as a result are more durable. Another advantage of using an LED array over CCFLs is that they usually have a longer functional lifetime. However, there are design challenges when using an LED array as a backlight for an LCD. Good uniformity is harder to achieve as the LEDs age with each LED possibly aging at a different rate. Also, the use of three separate light sources for red, green, and blue may cause the white point of the display to move as the LEDs age at different rates.
LEDs used for backlighting an LCD may be driven with DC voltage through a current limiting resistor. This approach is acceptable for many applications. When other considerations, (e.g. an extra bright display, low power consumption, or a backlight that can be controlled over a wide brightness range), are important, another method is often used.
A Pulse Width Modulation (PWM) method may have several advantages over the DC voltage method. A first advantage is that a brighter backlight may be achieved with a PWM method than the DC voltage method while using the same amount of power. For example, a LED backlight on an LCD using the DC voltage method LED may require a driving current for this display of 120 ma which produces a typical brightness of 50 NIT (a unit of measurement of the intensity of visible light, where 1 NIT is equal to one candela per square meter). If, instead of using the DC voltage method, a PWM method is used and five times the current, 600 ma, for ⅕ of the time is used, the average current is the same, 120 ma.
The average brightness of the LED would be the same if measured electronically. The difference is in the brightness perceived. The human eye has a certain amount of persistence. If exposed to a bright light the eye will “remember” the light for a short period of time. This allows us to view a motion picture or TV screen as a steady image when in fact it is flickering at 24 to 30 times a second. When an LED is flashed on brightly for a short time and then turned off, the eye “remembers” the light at the high brightness level. The result is that the perceived brightness of the backlight is closer to the high pulsed brightness than to the lower average DC brightness.
A PWM method may also be used to give a “normal” looking brightness level to the LCD but at a lower average current to save power. The average power may be cut by a factor of 30% to produce a given perceived brightness level.
The LED backlight brightness may also be varied while using the DC voltage method by varying the DC current to the LED's, but at low current the individual LED emitters become visible resulting in an uneven looking backlight.
Another use of the PWM method is to facilitate a wide range of brightness control for the LED backlight without an uneven looking backlight. By varying the duty cycle (pulse duration divided by the pulse period) of the controlling PWM waveform, a very wide range of brightness can be achieved while maintaining a very even appearing backlight.